1. Field of the Invention
The present invention relates generally to disk drive storage devices and more particularly to spin valve magnetic sensors used in disk drives.
2. Description of the Background Art
Disk drives provide the majority of nonvolatile storage in computer systems. A disk drive includes a disk upon which digital information is recorded and a magnetic sensor which reads back the recorded information. Spin valve sensors are commonly used as the readback sensor in most disk drives. A spin valve sensor typically has a single pinned layer or an antiparallel pinned substructure which is stabilized with an antiferromagnetic (AFM) layer. In addition there is a free layer which requires bias stabilization. The bias stabilization of the free layer can be provided with a set of hard magnets or by using a second AFM layer. Using a second AFM layer is advantageous when the sensor is to be used for narrow trackwidths. The second AFM layer can be used in an in-stack biasing arrangement or as part of a bias tab arrangement.
When using two AFM layers in a spin valve sensor it is necessary to initialize both layers and the directions of initializations are typically orthogonal to each other. The process of initialization is described in detail below.
One method of initializing the two AFM layers is to use different AFM materials which have different blocking temperatures. Two initialization steps at different temperatures are carried out. This method does not allow the use of the same AFM material to be used in both of the AFM layers.
Another method is to use the spin flop effect (described in detail below) to simultaneously initialize the two AFM layers by placing the sensor in a magnetic field chosen to give orthogonal initialization. The sensor must have at least one antiparallel (AP) pinned substructure in order to use the spin flop effect. The sensor is then heated above the blocking temperatures of the AFM materials in the presence of that field. One problem with this method is that the spin flop can result in a 90 degree orientation as often as a xe2x88x9290 degree orientation. Therefore the signal polarity of the sensor is not predictable. In most servo systems within disk drives it is important that the polarity of the readback signal is well defined. Accordingly the lack of control of signal polarity is undesirable. Another aspect of the previous method of initializing the spin flop sensor is that when using an AFM material with a high blocking temperature (e.g. PtMn and NiMn), heating the sensor above the blocking temperature can cause thermal damage.
Thus there is a need for using the spin flop effect to initialize a spin valve sensor having two AFM layers which allows for control of signal polarity, allows for use on the finished sensor, and does not place the sensor at risk to thermal damage.
In one embodiment of the present invention, the AFM layer adjacent to the AP pinned substructure in a spin valve sensor is pre-initialized. The spin flop effect is then utilized in an initialization. Another aspect of the present invention is that both the pre-initialization and the initialization may be accomplished by performing a thermal treatment of the sensor at a lower temperature than the blocking temperature of the AFM layers. A method embodying the present invention results in a sensor having two AFM layers wherein the AFM layers can be formed from substantially the same material; the polarity of the sensor is well controlled; and, there is much less risk of thermal damage to the sensor.
In order to use the spin flop effect in initializing a spin valve sensor, there must be an antiparallel substructure within the sensor. In one embodiment of the present invention, the AFM layer adjacent to one of the ferromagnetic layers in the antiparallel substructure is pre-initialized by performing a thermal treatment in the presence of a relatively high magnetic field. The temperature may be lower than the blocking temperature of the AFM material. An alternative is to pre-initialize utilizing a magnetic field with low magnitude. After the pre-initialization, the magnetic field is rotated and the magnitude of the field is chosen to maintain the pre-initialization state of the first AFM layer. Then a second thermal treatment is performed in the presence of the second magnetic field to initialize the second AFM layer. Because the second magnetic field is chosen to take advantage of the spin flop effect, the pre-initialized state of the first AFM layer does not substantially change.
In another embodiment of the invention the randomness in the initial directions of magnetization is greatly reduced by providing for a preferred direction of the magnetization by creating a uniaxial anisotropy in the constructed films. The sensor is then pre-initialized by being placed in a low magnetic field. The magnitude of the field is then increased to the optimal value to utilize the spin flop effect during an initialization.
Magnetic sensors manufactured according to an embodiment of the present invention thus have well defined signal polarity and have much less risk of thermal damage. Other aspects and advantages of the present invention will become apparent from the following detailed description which when taken in conjunction with the accompanying drawings, illustrate by way of example the principles of the invention.